Field of the Invention
The field of the present invention relates generally to propellers for aircraft. More particularly, the field of the present invention relates to a propeller having a plurality of composite propeller blades.
Description of Related Art
In aviation it is known to use propellers having one or more propeller blades to generate an airflow to which an airframe can react, for example, so as to generate a forward thrust to propel an aircraft. It is also known that such propellers may be of variable pitch such that the angle of attack of the propeller blades relative to an airflow can be adjusted in order to optimise propeller performance when operating at any particular speed.
Recently, improvements in propeller blade design have led to the increased use of composite materials to form the blades. The composite materials have weight advantages when compared to conventional metallic propeller blades.
However, whilst composite propeller blades have numerous advantages, the materials used to form them have to be shaped so as to provide generally smooth and gradual transitions in shape from an aerofoil section to a supporting blade root because the fibres that are used cannot be made to change direction abruptly without losing a substantial amount of their mechanical strength. This contrasts with metallic blades, and means that, generally, the non-aerofoil portion of composite blades is greater in length than for metallic blades of comparable aerodynamic performance.
Additionally, composite blades that are to be used in variable pitch propellers require a generally circular cross-sectioned root portion to engage with known pitch change bearings. Thus, for a given aerofoil size, a variable pitch propeller having composite blades will generally have an even greater diameter than a metallic-bladed equivalent because of the length of the transition from the aerofoil to the generally circular sectioned root. This results in the composite version of the propeller needing a relatively larger spinner size with an attendant increase in drag, as well as giving rise to design difficulties in matching the spinner to the nacelle of the aircraft when trying to optimise overall aircraft aerodynamic performance.